Gas guage problems

[Richardwa]

71 GMC 2500.
My gas gauge quit working, stayed at 1/2 tank. I unhooked the wire and it went way above full, about the 3 oclock position. I grounded the wire and the gauge went to empty so I figured it was a bad sending unit. When I hooked the wire back up the needle went to between 3/4 and full.
I bought and installed a new sending unit still the same problem. I took the sending unit out and hooked the wire up and grounded it, raise and lower the float and the gauge works as it should. Did the same with the old sending unit and it works as it should also.
I have tried running a ground wire to it and no change. When either sending unit is installed in tank the gauge reads between 3/4 and full.
There is only about 5 gallons of gas in it.
I also unhooked wire from fuse box and gauge went to 3 oclock.
I am running out of ideas here, hope you guys have some ideas.

Sorry for the long post but I wanted to be as specific as possible

[RustyPile]

On the back of the gauge, there is a resistor mounted between 2 of the posts.. The resistor can't be tested while it is mounted, it has to be removed. Using a voltmeter set to read ohms, test the resistor.. It should be ~ 85 - 90 ohms.. Any more or less, consider it bad..

[K10-Kansas]

Originally Posted by Richardwa

71 GMC 2500.
My gas gauge quit working, stayed at 1/2 tank. I unhooked the wire and it went way above full, about the 3 oclock position. I grounded the wire and the gauge went to empty so I figured it was a bad sending unit. When I hooked the wire back up the needle went to between 3/4 and full.
I bought and installed a new sending unit still the same problem. I took the sending unit out and hooked the wire up and grounded it, raise and lower the float and the gauge works as it should. Did the same with the old sending unit and it works as it should also.
I have tried running a ground wire to it and no change. When either sending unit is installed in tank the gauge reads between 3/4 and full.
There is only about 5 gallons of gas in it.
I also unhooked wire from fuse box and gauge went to 3 oclock.
I am running out of ideas here, hope you guys have some ideas.

Sorry for the long post but I wanted to be as specific as possible

Mine has similar behaviors. When I bought the truck, the unit didn't work. I pulled out the sending unit and it was obviously bad, the floater was half gone. I replaced theo entire unit and it reads a half tank when full and a full tank when 90% full. And then thereafter acts right as the week goes along. I think the unit is getting hung up inside the tank. Maybe your's is also?

[Palf70Step]

Double check the ground on the tank and gauge. Both have to be good for gauge to work correct. I've had to add another ground to my tanckand on another truck added a ground wire to the gauge to correct it.

[cj847]

On my truck the gauge issue was extra resistance in the fuse block. The fuse box was corroded which added resistance to the circuit.

[1970cstblazer]

Either bad tank grounding, or resistance in the wiring harness/connections.

[Richardwa]

Thanks for the info.
I know the tank ground is good because that is what I used to test the sending units.
I will check the wiring and fuse box for corrosion.
I am just having a hard time understanding why everything works fine until the sender is installed in tank.

[58 Special]

May have to adjust the angle the float is sitting. Bending the arm of the sender unit to fine tune the reading.

[KevinK7]

Gonna follow along, as my gauge stopped working for no reason. Haven't looked at it yet, but this thread may get me to

[damnyankee36]

If you're opening and grounding the circuit at the end of the wire that connects directly to the sender then the circuit from the sender to gauge is working properly.

What's interesting is how the gauge winds up being somewhere in the middle when everything is connected. It acts like both senders are stuck in mid-travel. But that seems unlikely.

Sure sounds like a ground issue...

[RustyPile]

Make absolutely certain that both sender float arms aren't hanging up in the same spot inside the tank..

[Chorner72]

I don’t mean to hijack this post, but I have a 1972 C15 and my gas gauge sits around 1/4 all the time. I can ground it out and the gauge moves as it should. I changed the sending unit last year and replaced my printed circuit on the back of the cluster. Still the same issues persists. Leads me to believe the resistor on the back of the gauge is the culprit. Where can a guy find a new resistor to buy? After reading this thread I can figure out what else the problem could be I am fairly certain you can only install the sending unit one way in these tanks, so I doubt the float is getting hung up. And help or suggestions are appreciated for where I can find a new resistor or if maybe that is not the problem. Thanks everyone.

[RustyPile]

Originally Posted by Chorner72

I don’t mean to hijack this post, but I have a 1972 C15 and my gas gauge sits around 1/4 all the time. I can ground it out and the gauge moves as it should. I changed the sending unit last year and replaced my printed circuit on the back of the cluster. Still the same issues persists. Leads me to believe the resistor on the back of the gauge is the culprit. Where can a guy find a new resistor to buy? After reading this thread I can figure out what else the problem could be I am fairly certain you can only install the sending unit one way in these tanks, so I doubt the float is getting hung up. And help or suggestions are appreciated for where I can find a new resistor or if maybe that is not the problem. Thanks everyone.

There is nothing "special" about the resistor.. It's a wire wound resistor on a ceramic piece designed to fit the posts on the back of the gauge.. Any flame proof axial (Leads on each end) 85 - 90 ohm 2 or 3 watt resistor will work.. Most electronic supply outlets will have these resistors in stock.. If you are unable to locate a source, Mouser Electronic Supply in the Dallas Ft/Worth area has them..

https://www.mouser.com/?utm_source=b...Brand%20Mouser

[Chorner72]

Originally Posted by RustyPile

There is nothing "special" about the resistor.. It's a wire wound resistor on a ceramic piece designed to fit the posts on the back of the gauge.. Any flame proof axial (Leads on each end) 85 - 90 ohm 2 or 3 watt resistor will work.. Most electronic supply outlets will have these resistors in stock.. If you are unable to locate a source, Mouser Electronic Supply in the Dallas Ft/Worth area has them..

https://www.mouser.com/?utm_source=b...Brand%20Mouser

Forgive my ignorance on these resistors. I do not see the ceramic ones on the site you provided, thank you for that by the way. Or were you saying to just use a different style of one? Thanks for your help on this.

[RustyPile]

Originally Posted by Chorner72

Forgive my ignorance on these resistors. I do not see the ceramic ones on the site you provided, thank you for that by the way. Or were you saying to just use a different style of one? Thanks for your help on this.

You won't find the exact replacement resistor that matches physically.. "Flame proof" is the type you want, not carbon composition resistors. Mounting the resistor is simple. Bend the wires into a loop around the posts, keeping a space between the resistor and the metal housing. Do not overtighten the nuts holding the resistor.

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